Myostatin, also known as growth/differentiation factor 8 (GDF-8), is a transforming growth factor-β (TGF-β) family member known to be involved in regulation of skeletal muscle mass. Most members of the TGF-β-GDF family are expressed non-specifically in many tissue types and exert a variety of pleitrophic actions. However, myostatin is largely expressed in the cells of developing and adult skeletal muscle tissue and plays an essential role in negatively controlling skeletal muscle growth (McPherron et al. Nature (London) 387, 83-90 (1997)). Recent studies, however, indicate that low levels of myostatin expression can be measured in cardiac, adipose and pre-adipose tissues.
The myostatin protein has been highly conserved evolutionarily (McPherron et al. PNAS USA 94:12457-12461 (1997)). The biologically active C-terminal region of myostatin has 100 percent sequence identity between human, mouse, rat, cow, chicken, and turkey sequences. The function of myostatin also appears to be conserved across species as well. This is evident from the phenotypes of animals having a mutation in the myostatin gene. Two breeds of cattle, the Belgian Blue (Hanset R., Muscle Hypertrophy of Genetic Origin and its Use to Improve Beef Production, eds, King, J. W. G. & Menissier, F. (Nijhoff, The Hague, The Netherlands) pp. 437-449) and the Piedmontese (Masoero, G. & Poujardieu, B, Muscle Hypertrophy of Genetic Origin and its Use to Improve Beef Production., eds, King, J. W. G. & Menissier, F. (Nijhoff, The Hague, The Netherlands) pp. 450-459) are characterized by a “double muscling” phenotype and increase in muscle mass. These breeds were shown to contain mutations in the coding region of the myostatin gene (McPherron et al. (1997) supra). In addition, mice containing a targeted deletion of the gene encoding myostatin (Mstn) demonstrate a dramatic increase in muscle mass without a corresponding increase in fat. Individual muscles of Mstn−/− mice weigh approximately 100 to 200 percent more than those of control animals as a result of muscle fiber hypertrophy and hyperplasia (Zimmers et al. Science 296, 1486 (2002)).
Administration of myostatin to certain strains of mice has been shown to create a condition similar to muscle wasting disorders found associated with cancer, AIDS, and muscular dystrophy, for example. Myostatin administered as myostatin-producing CHO cells to athymic nude mice resulted in a wasting effect with a high degree of weight loss, a decrease of as much as 50% of skeletal muscle mass in addition to fat wasting, and severe hypoglycemia (Zimmers et al. supra).
Loss of myostatin appears to result in the retention of muscle mass and reduction in fat accumulation with aging. It has been shown that age-related increases in adipose tissue mass and decrease in muscle mass were proportional to myostatin levels, as determined by a comparison of fat and muscle mass in Mstn+/+ when compared with Mstn−/− adult knockout mice (McFerron et al. J. Clin. Invest 109, 595 (2002)). Mstn−/− mice showed decreased fat accumulation with age compared with Mstn+/+ mice.
In addition myostatin may play a role in maintaining blood glucose levels and may influence the development of diabetes in certain cases. It is known that, for example, skeletal muscle resistance to insulin-stimulated glucose uptake is the earliest known manifestation of non-insulin-dependent (type 2) diabetes mellitus (Corregan et al. Endocrinology 128:1682 (1991)). It has now been shown that the lack of myostatin partially attenuates the obese and diabetes phenotypes of two mouse models, the agouti lethal yellow (Ay) (Yen et al. FASEB J. 8:479 (1994)), and obese (Lepob/ob). Fat accumulation and total body weight of the Ay/a, Mstn−/− double mutant mouse was dramatically reduced compared with the Ay/a Mstn+/+ mouse (McFerron et al., (2002) supra). In addition, blood glucose levels in the Ay/a, Mstn−/− mice was dramatically lower than in Ay/a Mstn+/+ mice following exogenous glucose load, indicating that the lack of myostatin improved glucose metabolism. Similarly Lepob/ob Mstn−/− mice showed decreased fat accumulation when compared with the Lepob/ob Mstn+/+ phenotype.
Therefore, there is considerable evidence from the phenotypes of over-expressing and knockout animals that myostatin may play a role in contributing to a number of metabolic disorders including disorders resulting in muscle wasting, diabetes, obesity and hyperglycemia.